Control system for positioning compressor inlet guide vanes

ABSTRACT

A control system is disclosed for optimizing the transient response of a gas turbine engine by controlling the variable positioning of compressor inlet guide vanes. The system employs a normal mode schedule which schedules relatively closed inlet guide vane settings at low compressor speeds and relatively open inlet guide vane settings at high compressor speeds. The system further employs an alternate mode schedule that schedules inlet guide vane settings that are more closed at low compressor speeds than those scheduled by the normal mode schedule. Control logic is provided for rapidly moving the inlet guide vanes from the more closed settings of the alternate mode schedule to settings which are more open than those which are schedule by the normal mode schedule, during an acceleration from low engine power levels. The control logic is further configured to command the inlet guide vanes back to the normal mode schedule as the acceleration nears completion.

CROSS-REFERENCE TO RELATED APPLICATION

The subject application is a divisional application of U.S. patentapplication Ser. No. 09/974,585 filed Oct. 10, 2001.

GOVERNMENT RIGHTS STATEMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms ofDAAH10-99-2-0005, awarded by the U.S. Department of the Army.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates to the operation of gas turbine engines,and more particularly, to a control system for positioning compressorinlet guide vanes on a gas turbine engine in such a manner that producesoptimized engine transient response.

2. Background of the Related Art

Gas turbine engine for use in rotary wing aircraft, such as helicopters,are typically designed with variable compressor inlet guide vanes (IGVs)which are used in such a manner so as to minimize engine fuel burn.FIGS. 1a and 1 b are compressor performance maps illustrating typicalcompressor performance characteristics as a function of IGV setting,compressor speed and inlet airflow. The variable positioning of theinlet guide vanes allows for optimization of compressor efficiency for adesired engine power level and given air inlet conditions atsteady-state conditions. This in turn results in minimized engine fuelburn which is of particular importance at aircraft cruise powerconditions.

Modern state-of-the-art digital control systems typically control thepositioning of compressor IGV's according to a normal mode schedulewhich is illustrated in FIG. 2. The normal mode schedule is typicallyderived by engine performance engineers and is optimized solely forsteady-state engine performance. It would be beneficial however toprovide a control strategy for positioning inlet guide vanes duringrapid engine accelerations and decelerations in an effort to optimizeengine transient response.

SUMMARY OF THE INVENTION

The subject invention is directed to a control system controlling thevariable positioning of the compressor inlet guide vanes of a gasturbine engine. The system employs a normal mode schedule or map whichschedules relatively closed inlet guide vane settings at low compressorspeeds and relatively open inlet guide vane settings at high compressorspeeds. The normal mode schedule is adapted and configured to optimizeengine performance by minimizing fuel burn during steady-state engineoperation.

The system further employs an alternate mode schedule or map whichschedules inlet guide vane settings that are more closed at lowcompressor speeds than those scheduled by the normal mode schedule. Thealternate mode schedule is adapted and configured to optimize transientengine response during fast engine acceleration.

Control logic is provided for rapidly moving the inlet guide vanes fromthe more closed settings of the alternate mode schedule to settings thatare more open than those scheduled by the normal mode schedule, duringan acceleration from low engine power levels. The control logic isfurther configured to smoothly command the inlet guide vanes back to thesettings of the normal mode schedule as the engine acceleration nearscompletion.

These and other aspects of the subject invention will become morereadily apparent to those having ordinary skill in the art from thefollowing detailed description of the invention taken in conjunctionwith the drawings described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those having ordinary skill in the art to which the subjectinvention pertains will more readily understand how to employ thecontrol system of the subject invention, preferred embodiments thereofwill be described in detail hereinbelow with reference to the drawings,wherein:

FIGS. 1a and 1 b are compressor performance maps illustrating typicalcompressor performance characteristics as a function of IGV setting,compressor speed and airflow;

FIGS. 2a and 2 b illustrate a conventional open-loop steady-stateschedule (hereinafter the “normal mode schedule”) for positioning thecompressor inlet guide vanes of a gas turbine engine;

FIG. 3 illustrates the control logic of the subject invention whichincorporates the conventional normal mode schedule of FIG. 2b and analternate mode schedule for fast engine acceleration;

FIG. 3a illustrates the alternate mode schedule overlaid with the normalmode schedule to illustrate the relative differences therein; and

FIG. 4 illustrates simulation results of a gas turbine engine rapidacceleration with the normal mode IGV control and with alternate modeIGV control.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS

As noted above, modern state-of-the-art digital control systems directthe positioning of compressor inlet guide vanes according to a normalmode schedule which is illustrated in FIG. 2. The normal mode schedulereceives an input signal indicative of the corrected compressor speedNL_(c) and outputs a signal indicative of the IGV position commandedfrom digital engine control (IGVDEMAND). In accordance with the normalmode control schedule, the inlet guide vanes are set at an angle ofabout 30° until NL_(c) approaches about 66%. Thereabout, in accordancewith the normal mode schedule, the inlet guide vane angles move to moreopen settings which reach a maximum open position at an angle of about100° corresponding to the NL_(c) approaching about 103%.

The subject invention optimizes engine transient response through theuse of an alternate IGV control strategy. In this strategy, the normalmode IGV schedule is maintained for high engine power levels to minimizeor otherwise optimize fuel burn during normal engine operation, such asduring steady-state flight conditions. At lower engine power levels, thealternate mode IGV schedule is designed to maintain high compressorspeed while compromising on operating efficiency and engine fuel burn.At the lower engine power levels, the alternate mode schedule isconsidered “more closed” than the normal mode schedule.

The subject invention further includes transient control logic for useduring fast engine accelerations from low engine power. The controllogic, which is described in more detail hereinbelow with reference toFIG. 3, is configured to rapidly move the inlet guide vanes from the“more closed” position of the alternate mode schedule to settings whichare “more open” than the normal mode schedule. (See FIG. 4b) Then, asthe engine acceleration nears completion, the control logic smoothlycommands the inlet guide vanes back to their normal mode schedule.

Referring to FIG. 3, there is illustrated the IGV control strategy orlogic of the subject invention. As configured, the normal mode IGVschedule 10 is selected when the FAST_ACCEL_FLAG is set equal to FALSE.Alternatively, when the FAST_ACCEL_FLAG is set equal to TRUE, the IGVcontrol logic of the subject invention is invoked, and the alternatemode IGV schedule 20 is employed. This results in faster engineaccelerations than are provided under the normal mode schedule.

In the fast engine acceleration mode, the alternate IGV schedule 20schedules “more closed” IGV's at low engine speeds than those which arescheduled by the normal mode schedule at low engine speeds, asillustrated in FIG. 3a. For example, the normal mode schedule 10provides that the inlet guide vanes are set at an angle of about 30°until the corrected compressor speed approaches about 66%. In contrast,the alternate mode schedule 20 provides that the inlet guide vanes areset at an angle of about 20° until the corrected compressor speedapproaches about 73%.

With continuing reference to FIG. 3, based upon an input signalindicative of the corrected compressor speed NL_(c), the output of thealternate IGV schedule 20 is the IGV setting IGVSP1. The difference inIGV setting between the alternate fast acceleration mode schedule(IGVSP1) and the normal mode schedule (IGVSSNOM) is then computed atsumming junction 30 to obtain (DIGVSP1). An acceleration ratio(NDOTRATIO) is then computed at division block 40 based on the ratio ofthe acceleration rate demand (NDOT_DEMAND) which defines the numeratorof the ratio and the maximum acceleration limit (NDOT_ACCEL_LIMIT) whichdefines the denominator of the ratio.

This ratio is then used to compute an IGV anticipator value (DIGVMOD)using a look-up table 50. The anticipator is zero (0) for lowNDOTRATIO's and is greater than one (1.0) when NDOTRATIO nears 1.0. Theanticipator value DIGVMOD is then multiplied with DIGVSP1 atmultiplication block 60 to produce the transient IGV demand DIGVSPTR.The demanded transient IGV is then added to the scheduled IGVSP1 atsumming junction 70 to produce the final command signal from the fastacceleration mode IGV logic, which is IGVSPOIL.

Referring to FIG. 4, there is illustrated simulation results of a gasturbine engine rapid acceleration with the normal mode IGV control andwith the alternate IGV control, with respect to free turbine speed, IGV,high pressure spool speed (NH), and low pressure spool speed (NL). Asillustrated, in accordance with the alternate mode schedule, as theengine is accelerated from a low power level, the inlet guide vane anglerapidly increases from the more closed setting of about 20° to the moreopen setting of about 60° in under one second. Consequently, there is arapid output of engine power during acceleration.

The gas turbine engine used for this simulation was a turboshaft enginewith a two-spool gas generator and free turbine driving a helicopterrotor system. The low pressure compressor was equipped with variableinlet guide vanes. The acceleration rates were the same for both tracesin FIG. 4 and the improvement in engine power response is due solely tothe alternate IGV control strategy.

It is envisioned that the control strategy of the subject invention canbe employed in any gas turbine application equipped with inlet guidevanes where transient engine performance is important. For example, thecontrol strategy of the subject invention may be employed in gasturbines powering helicopter rotor systems, electrical power generatingsystems or propeller drive systems. It is also envisioned that thecontrol strategy of the subject invention, which results in more rapidpower output during gas turbine accelerations, may be applied to gasturbine decelerations. In such an instance, more rapid engine powerdrops can be achieved. Further, the control strategy of the subjectinvention can be employed in any gas turbine application whether singleor multiple gas generator spools are employed

Although the control system of the subject invention has been describedwith respect to preferred embodiments, those skilled in the art willreadily appreciate that changes and modifications may be made theretowithout departing from the spirit and scope of the present invention asdefined by the appended claims.

What is claimed is:
 1. A method of controlling variable positioning ofcompressor inlet guide vanes of a gas turbine engine comprising thesteps of: a) providing a normal mode schedule which schedules relativelyclosed inlet guide vane settings at low compressor speeds and relativelyopen inlet guide vane settings at high compressor speeds; b) providingan alternate mode schedule which schedules inlet guide vane settingsthat are more closed at low compressor speeds than those scheduled bythe normal mode schedule; c) rapidly moving the inlet guide vanes fromthe more closed settings of the alternate mode schedule to settingswhich are more open than those that are scheduled by the normal modeschedule during an acceleration from low engine power levels; and d)commanding the inlet guide vanes back to the normal mode schedule as theacceleration nears completion.
 2. A method as recited in claim 1,further comprising the step of invoking the alternate mode schedule forfast engine acceleration.